The RNC Can Legally Dump Donald Trump But It Has to Act Fast (super barf alert!) http://www.thedailybeast.com/articles/2016/08/03/the-rnc-can-legally-dump-donald-trump-but-it-has-to-act-fast.html
The writer of this article consults a friend who is a law professor and "expert" in election law. The good professor goes through the various scenarios where Trump is no longer the candidate and then posits the best replacement would be PAUL RYAN!
Of course, as likely next Tuesday, Ryan will not be a viable candidate for anything.
But beyond that, if Trump is replaced or quits, the likelihood of the replacement winning is slim, at best, as angry supporters desert the party and either stay home or vote the libertarian candidate.